The present invention generally relates to an electrostatic dissipative (ESD) composition for preparing films and/or coating substrates. More particularly, the present invention relates to a method of forming a transparent, water-based, electrostatic dissipative composition, to a method of using the composition to form an electrostatic dissipative film on a substrate, and to a method of using the composition to form an electrostatic dissipative article.
Many plastics are prized for their insulating ability that is derived from the relatively high electrical surface resistance of these plastics. Unfortunately, high electrical surface resistance is an undesirable property in some applications since the high electrical surface resistance will typically allow build-up or accumulation of static electricity charge on outer surfaces of the plastic. This static electricity charge accumulation is problematic in and of itself because the accumulated charge may attract migrant dust particles and cause the surface of the plastic to collect dust.
Furthermore, under certain circumstances which commonly occur, the accumulated static electricity charge may discharge to adjacent articles. Electrical components, such as computer chips, and electronic products, such as computers, televisions, and other industrial or consumer goods are frequently wrapped in plastic, such as by shrink wrapping, due to the relatively low cost of such packaging methods. However, this method of packaging electronic components and articles has not realized its full potential because of the damage and/or destruction that discharges of static electricity may cause to packaged electronic components and articles.
There have been numerous attempts to develop acceptable coatings and compositions for plastics that are capable of minimizing the amount of static electricity charge development on the surface of plastic packaging materials. However, despite advancing the knowledge base with regard to static dissipative materials, these efforts have not been entirely satisfactory for a variety of different reasons. One approach involves incorporation of anti-static material into the polymer matrix of the plastic packaging materials. Quaternary ammonium compounds and amine-type compounds are known internal anti-static materials that function by attracting moisture from the surrounding environment to create a conductive layer of surface moisture on the plastic packaging material. However, these types of internal anti-static compounds require a relative humidity of at least about 15% in the surrounding environment to perform effectively. Thus, the practical applications of these types of internal anti-static materials are limited, since packaged electronic components and products are frequently shipped from, through, or to arid parts of the world with relative humidities of less than 15%. Furthermore, conductive surface moisture layers that are created by these internal anti-static compounds quickly lose their effectiveness when conditions change that cause evaporation of the moisture and under conditions when the package is jostled or otherwise subject to surface contact with other articles being shipped, with conveying equipment, or with the shipping equipment itself.
Other traditional types of internal anti-static materials include conductive fillers, such as carbon black, metallic powder, fibers, and inherently dissipative polymers (LDP), such as polypyrroles and polyanilines. Traditional conductive fillers, such as carbon black, metallic powder, and metallic fibers are problematic, because each of these materials reduces the transparency of the plastic packaging material. For aesthetic reasons, as well as practical reasons, it is desirable that packaging materials be highly transparent. For example, a high level of transparency is needed to permit inspection of the goods in the package through the packaging material using a variety of different methods, including, but not limited to, automated bar code inspection, without the need to physically open the package.
Inherently dissipative polymers (IDP), such as the polypyrroles and polyanilines, may also create transparency problems when incorporated at the relatively high concentrations needed to effect adequate static discharge properties in the plastic packaging materials. Additionally, IDP polymers are relatively high cost, which limits use of IDP polymers to packaging only the most valuable and costly electronic components and products. Furthermore, since IDP polymers typically are thermally stable only up to relatively low temperatures of about 200° C., or so, the physical ability to internally incorporate IDP polymers into plastic packaging materials is severely limited. For example, at the hot temperatures typically required during conventional polymer processing, such as extrusion and injection molding, many anti-static IDP polymers are incapable of withstanding the high temperatures and are damaged or destroyed, thereby rendering the IDP polymers that are incorporated useless for purposes of providing electrostatic discharge properties to the plastic packaging materials.
As an alternative to internally mixing or filling a plastic packaging material with an electrostatic dissipative substance, a film of an electrostatic dissipative material may be applied onto one or both sides of the plastic packaging material. A number of attempts have been made to create such a packaging material that incorporates a film of electrostatic dissipative material onto a plastic substrate. Though these various approaches have advanced the knowledge base with respect to electrostatic dissipative packaging materials, none of these approaches have optimally addressed and achieved a satisfactory balance of the various properties that such electrostatic dissipative packaging should have.
Aside from sometimes being difficult to achieve individually, achieving an optimal combination of these different properties is a very difficult challenge, since several of these properties are based upon variables that conflict with each other. First, both the electrostatic dissipative film or coating and the electrostatic dissipative packaging material should desirably be economical to produce. Thus, the electrostatic dissipative substance that is incorporated preferably should be a low cost substance, incorporated at a relatively low concentration, or be both low in cost and incorporated at a relatively low concentration. Additionally, the ESD coating should preferably be highly transparent and without color. These properties are desirable to allow for quick visual inspection of the packaged contents and machine-based inspection of the contents, such as automated bar code inspection, without the need to open the package.
Also, the completed electrostatic dissipative film or coating should preferably have a relatively high surface resistivity that minimizes or prevents charge accumulation in the ESD coating or film and in the ESD packaging product. The surface resistivity of the ESD coating should preferably be adequate to minimize or prevent dust buildup on the packaging material and should also preferably be adequate to prevent a level of current discharge that either damages the packaged goods or that causes discomfort or fear in persons touching the packaged product.
Furthermore, the ESD coating or film should preferably have a very low level of ion contamination and a relatively neutral pH ranging from about 6 to about 9 standard pH units, especially when used for packaging electronic components and articles. Tramp ions, such as chloride, sulfate, phosphate, fluoride, nitrite, bromide, nitrate, and silicon, that escape from the ESD coating or film of the ESD packaging material are often highly corrosive and detrimental to electronic components that are encapsulated in the ESD packaging material. The relatively neutral pH of the ESD composition will further help prevent corrosive damage to products encapsulated in the ESD packaging material. This eliminates ESD substances, such as polyanilines and polypyrroles, that require a low pH environment to maintain the surface resistivity of the conductive polymer.
Packages that are being shipped frequently encounter a wide variety of different atmospheric conditions, such as widely changing levels of relative humidity and widely varying temperatures. Therefore, the ESD coating or film should preferably also experience only minimal changes in surface resistivity when going from low temperature to high temperature environments, and vice versa, and likewise should preferably also experience only minimal changes in surface resistivity when passing from low relative humidity to high relative humidity environments, and vice versa.
Besides the finished ESD or film, properties exhibited during formation of the ESD coating or film are also important. For example, environmental and safety considerations dictate that the ESD composition should preferably contain little if any concentration of volatile organic compounds (VOCs). Also, the ESD-composition that is used to form the ESD coating or film will preferably be capable of easy application to the plastic substrate of the packaging material and formation of uniformly thin ESD coatings or layers on the substrate. This is a difficult proposition, since the ESD substances in the ESD composition should also remain uniformly dispersed within the composition over relatively long periods of time, such as 30 to 60 days or more. Also, the ESD substance should not flocculate, agglomerate, or precipitate to any significant degree in the ESD composition over these relatively long periods of time of 30 to 60 days or more.
Steric and electrostatic properties are typically imparted to individual ESD particles to help maintain this desired stability of the ESD substance in the ESD composition. However, the steric and electrostatic effects pose significant barriers to uniform film formation once the ESD composition is spread on the substrate and the suspending water and/or solvents of the composition are evaporated. Thus, what is needed to maintain a uniform dispersion and suspension of the ESD substance in the ESD composition is deleterious for uniform film formation due to impedance of uniform intermixing of the ESD substance in the coating during and after removal of water and hydrophillic solvents. Thus, these competing variables of uniform dispersability and suspendability in the ESD composition along with achievement of uniform coating or film thickness are inherently inconsistent with each other and present complex competing variable problems.
The applied ESD coating should preferably also possess a relatively fast cure time on the order of a minute or less at relatively low temperatures to support fast, automated ESD composition application and ESD coating or film formation. Finally, the ESD coating or film should also posses a variety of different properties that permit and support post-application processing. For example, after application of the ESD composition and evaporation of water and any solvent, the in-process ESD coating or film should beneficially be capable of surviving thermoforming operations without losing any significant degree of surface resistivity or transparency. Thermoforming is typically accomplished with pressing or vacuum equipment that incorporates heating devices to make the in-process ESD coating or film soft and pliable before the coating or film is deformed by a plunger or via vacuum in a mold. Thus, the in-process ESD coating or film should preferably have certain mechanical properties to survive the conditions encountered during thermoforming.
For example, the in-process ESD coating should preferably have a modulus of elasticity that is in the rubbery range at the thermoforming temperature to accommodate the large deformations that are imposed by deep drawing during the thermoforming process. Thus, the glass transition temperature (TV of the in-process ESD coating or film should beneficially be less than the temperature encountered during thermoforming. Also, the in-process ESD coating or film should preferably have adequate tensile strength and an adequate degree of elongation at break to avoid damaging the ESD coating or film during the deformation that occurs when thermoforming. The ESD coating or film should also remain adhered to the plastic packaging material or substrate without delamination over relatively long periods of time such as 30 to 60 days, or more. Consequently, the completed ESD coating or film should have a thermal expansion coefficient that is compatible during the conditions of use, storage, and shipping with any polymer substrate to which the ESD coating or film is bonded.
Thus, there are a number of different properties, including some properties that compete with other properties, that should be possessed by the ESD coating composition, the ESD coating or film, or by the in-process ESD coating or film during transformation from the ESD composition to the ESD coating or film. None of the presently known or existing ESD coating compositions support optimal achievement of these various properties. Therefore, despite an abundance of work on ESD coatings and compositions, a need still remains unfilled for an improved ESD coating composition that possesses an optimal-combination of the variety of different properties discussed above and optimally achieves the combination of desirable properties discussed above during and after formation of ESD coatings and films. The present invention provides a solution that achieves these beneficial properties in both ESD compositions and ESD coatings.